Latin name

Carcharhinus obscurus

Other name

Carcharhinus obscurus

Identification

The Dusky Shark has a slender, streamlined body with a broad, rounded snout. The skin folds in front of the nostrils are barely developed. The medium-sized round eyes are equipped with a blinking membrane. The mouth is very short, with thin furrows at the corners. The mouth has 13-15 (usually 14) rows of teeth on each side of both jaws. The upper teeth are broad, triangular, set at a slight angle and strongly serrated on the edges; the lower teeth are narrower and set upright, with shallower serrations on the edges. This shark has five pairs of fairly long gill slits. The placoid scales are lozenge shaped, the scales are set close together, each with five horizontal teeth. 

Features of fish fins

The large pectoral fins are approximately 1/5th of the length of the shark's body, sickle shaped with pointed tips. The first dorsal fin is moderately sized and not distinctly crescent-shaped, with a pointed tip and a strongly concave posterior margin. The base of the fin lies above the free posterior tips of the pectoral fins. The second dorsal fin is much smaller than the first and lies opposite the anal fin. There is a ridge between the first and second dorsal fins. The caudal fin is large and high, with a well-developed lower lobe and a ventral notch below the tip of the upper lobe.

Fish colouring

Bronze to bluish-grey in colour, with a white belly and a white stripe along the sides, which may be faint. The fins, especially the lower part of the pectoral fins and the lower lobe of the caudal fin, are darker at the edges. This characteristic colouration is more pronounced in juvenile sharks.

Distribution

The Dusky Shark is found worldwide in tropical and warm temperate waters. Its range is extensive, although intermittent. In the western Atlantic these sharks are found from Massachusetts and George's Bank to the Gulf of Mexico and from Nicaragua to the southern coast of Brazil. In the eastern Atlantic they are caught in the western Mediterranean, off the Canary Islands, Cape Verde, Senegal, Sierra Leone and probably elsewhere. In the Indian Ocean they inhabit the coastal waters of South Africa, Mozambique and Madagascar. There are occasional reports of these sharks in the Arabian Sea, the Bay of Bengal and possibly the Red Sea. In the western Pacific they are found from Japan to the states of Queensland and New South Wales, Australia. In the eastern Pacific, their range includes the coast of the American continent from southern California to Chile. Information about their presence in the north-eastern and central-eastern Atlantic is probably incorrect and refers to Galapagos sharks. Mitochondrial DNA and microsatellite studies have shown that sharks from Indonesian and Australian waters belong to separate populations.

Habitat

They inhabit the continental coastline from the surf zone to the outer edge of the continental shelf and adjacent marine waters, sharing space with more specialised relatives. A tagging study in the northern Gulf of Mexico found that these sharks spend most of their time at depths of 10-80 m, occasionally descending below 200 m. This species is known to be capable of reaching depths of 400 metres. It prefers water temperatures of 19-28°C and avoids low salinity areas such as estuaries.

Size

One of the largest members of the genus, usually reaching 3.2 m in length and 160-180 kg in mass. The maximum recorded length and mass are 4.2 m and 347 kg respectively. Females grow larger than males. Maximum lifespan is 40-50 years or more.

Behavior

They migrate up to 3800 km. Off the coast of North America, they move north in summer and towards the equator in winter. In South African waters, young females and males of 0.9m swim south and north of the KwaZulu-Natal birthplace and join the adults after a few years along unknown routes. Juveniles also spend spring and summer in the surf zone and autumn and winter in the open sea. When they reach 2.2 metres in length, they begin north-south migrations between KwaZulu-Natal in winter and the Western Cape in summer. Larger sharks, over 2.8 metres, migrate as far south as Mozambique. Off the coast of Western Australia, adult and juvenile sharks swim up the coast in summer and autumn, staying away from shallow bays and coves where newborns are born.

As predators at the top of the food pyramid, these sharks tend to be smaller in number than other sharks that share their range. However, there are places where aggregations of these sharks, especially juveniles, can be found. Adult sharks often accompany ships far from land, such as along the Cape Needle Current. Studies conducted at the mouth of the Cape Fear River in North Carolina reported an average speed of 0.8 km/h. 

Food and feeding habits

It is a versatile predator that feeds on a wide range of marine life at all levels of the water column. Large individuals can consume more than 1/10 of their own body weight in a single meal. The bite force of a 2 metre long dark shark is 60kg per 2mm² (the area of the tip of the tooth). This is the highest reliably measured for any shark. Dense aggregations of juvenile dark sharks have been recorded in the Indian Ocean in areas of high food concentration.

The diet of the dark shark includes pelagic bony fish, squid and invertebrates including cephalopods, crustaceans, molluscs and starfish. Very rarely, large individuals will eat sea turtles, marine mammals (usually as carrion) and human waste.

Reproduction

They are viviparous; developing embryos are nourished by a placental connection to the mother, formed by an empty yolk sac. Pregnancy lasts up to 22-24 months and females give birth once every three years. There are 3 to 16 newborns in a litter, the average number is 6-12, the number of offspring does not depend on the size of the female. Newborns are 0.7-1.0 m long; their size is directly related to the size of the mother and decreases with litter size. Evidence suggests that females may determine the optimal size at which to give birth to increase the chances of survival in response to environmental conditions. In addition, aculeates' livers weigh 1/5 of their body weight at birth, and they use the nutrients stored in the liver until they learn to hunt independently. They are one of the slowest growing shark species. They do not reach sexual maturity until they are large: depending on the region, males reach maturity at 2.8-3m at the age of 18-23 years, and females at 2.6-3.1m at 17-32 years. Annual growth is 8-11 cm during the first five years of life. 

Fishing

Targeted commercial fisheries off eastern North America, southwest Australia and eastern South Africa. In south-western Australian waters.  They are caught using bottom-set nets, which catch almost exclusively juvenile sharks under three years of age, including 18-28% of all newborns in their first year of life. This species is valued for sport fishing. 

Relationship with a person

Traumatogenic. Dusky sharks are considered potentially dangerous to humans due to their large size, although little is known about their behaviour towards humans underwater. Young sharks adapt well to life in captivity in aquariums. This shark is one of the most sought-after shark species on the market because of its large fins with many internal rays (ceratotrichia), which are used to make a popular soup. In addition, the meat is sold fresh, frozen, dried, salted or smoked, and the skin and fat are used.